The present invention generally relates to radio transceivers, and more specifically to portable battery-powered radio transceivers for data communications.
The frequency generating section of a radio transceiver typically includes a frequency synthesizer in a phase-locked loop. Although the transceiver transmits and receives on the same channel frequency, the phase-locked loop must provide one frequency for the transmitter portion when the transceiver is in the "transmit" mode and another frequency, offset by the intermediate frequency (IF), when the transceiver is in the "receive" mode. Thus, when the transceiver is switched from the receive mode to the transmit mode, and vice versa, the frequency synthesizer must be reset and the phase-locked loop must regain synchronization.
For a simple phase-locked loop circuit having both high frequency stability and low noise, the response time of the phase-locked loop for regaining synchronization is typically on the order of 10 milliseconds. For data communications applications, however, the transmitter portion of the transceiver must be able to respond much more rapidly. The response time of a simple phase-locked loop is too slow for most data communication applications in which the transmitter must respond in a few hundred microseconds or less.
Phase-locked loops with rapid response times can be constructed, but these circuits are more complex and therefore require additional components and consume more power. This is obviously less desirable for portable transceivers which are battery powered.